Available online at www.rilem.net Materials and Structures 38 (January-February 2005) 121-126 Properties of bituminous mixtures at low temperatures and relations with binder characteristics F. Olard 1, H. Di Benedetto 2, A. Dony 3 and J.-C. Vaniscote 4 (1) Appia, Direction de la Recherche et du Développement, France Previously at (2) (2) Département Génie Civil et Bâtiment (URA CNRS 1652), École Nationale des TPE, France (3) Eurovia, Centre de Recherche, France (4) Appia, Direction Technique, France Received: 30 August 2003; accepted: 4 March 2004 ABSTRACT Within the framework of a partnership between the Département Génie Civil et Bâtiment of the ENTPE, Eurovia and Appia, a research work including a large experimental campaign on the thermomechanical behavior of bituminous materials at low temperatures is proposed. The aim is to establish the links between the characteristics of the binder and the properties of bituminous mixes at low temperatures. Four different bitumens have been used with one type of aggregate and grading. The low temperature behavior of binders was evaluated with three fundamental tests: the complex modulus determination, the Bending Beam Rheometer and the tensile strength at a constant strain rate and constant temperatures. The thermomechanical behavior of bituminous mixes has been studied by performing complex modulus tests, measurements of the coefficient of thermal dilatation and contraction, tensile tests at constant temperatures and strain rates, and Thermal Stress Restrained Specimen Tests. The results are analyzed considering a rational approach. Some pertinent links between binders and mixes properties are established. Characteristics which are pertinent and discriminating enough with regard to the thermal cracking of bituminous mixes at low temperatures are presented. 1359-5997 2004 RILEM. All rights reserved. RÉSUMÉ Dans le cadre d un partenariat entre le Département Génie Civil et Bâtiment de l École Nationale des TPE, Eurovia (groupe VINCI) et Appia (groupe EIFFAGE), une vaste campagne de recherche sur le comportement thermo-mécanique des matériaux bitumineux a été menée. Le but de cette étude est d établir les liens existants entre les caractéristiques du liant et les propriétés de l enrobé à basses températures. Quatre bitumes différents et une seule formulation d enrobé ont ici été étudiés. Le comportement à basse température des bitumes a été évalué avec trois tests fondamentaux : i) l essai de module complexe, ii) l essai de fluage au rhéomètre BBR, iii) l essai de traction directe SHRP à vitesse de déformation constante et températures constantes. Le comportement thermo-mécanique des enrobés bitumineux a été étudié en réalisant i) des essais de module complexe, ii) des mesures du coefficient de dilatation-contraction thermique, iii) des essais de traction à vitesses de déformation constantes, ainsi que iv) des essais de retrait thermique empêché (TSRST). A partir des résultats obtenus, des liens pertinents entre les propriétés des liants et des enrobés, et des caractéristiques suffisamment discriminantes au regard des propriétés à basse température des enrobés sont mis en évidence. 1. INTRODUCTION The different types of bitumen behavior can be simply illustrated according to the strain amplitude ( ) and the temperature (T), at a given strain rate. Fig. 1 points up: - the fragile and ductile domains, where the tensile strength p can be measured, - the fragile failure, characterized by the fracture toughness K c, Editorial Note Presented at the 6 th International RILEM Symposium on Performance Testing and Evaluation of Bituminous Materials (PTEBM'03), held on 14 th -16 th April 2003, in Zurich, Switzerland, this paper was selected as an outstanding communication and peer-reviewed by the Scientific Committee of the Journal Materials and Structures. Mr. Hervé Di Benedetto participates in RILEM TCs 182-PEB Performance testing and evaluation of bituminous materials, ATB Advanced testing and characterization of bituminous materials and CAP Cracking in Asphalt Pavements. 1359-5997 2004 RILEM. All rights reserved. doi:10.1617/14132
122 F. Olard et al. / Materials and Structures 38 (2005) 121-126 - the linear elastic behavior, characterized by the moduli E et G, - the linear viscoelastic behavior, characterized by the complex moduli E* and G*, - the purely viscous (Newtonian) domain, characterized by the viscosity, - for strains of a few percent, the domain where the behavior is highly non-linear. A bituminous mix has also a complex temperature-sensitive behavior. Its response to a given loading is strongly dependent on temperature and loading path. Concerning bituminous mixes, four main typical behaviors can be identified according to the strain amplitude ( ) and the number of applied cyclic loadings (N) (Fig. 2). The scope of this paper is confined to the characterization of the linear viscoelastic properties (in the small strain amplitude domain) and the failure properties (in the highly non-linear domain) of bituminous materials when considering a small number of loadings, at low temperatures. Furthermore, a rational approach needs that the binders and mixes properties are compared in the same domain of behavior so that erroneous conclusions may not be drawn. Fig. 3 sums up the overall view of this study. Fig. 1 - Typical behaviors observed on bitumens [1]. Fig. 2 - Typical mix behavior domains [2]. 2. MATERIALS Two pure bitumens (10/20 and 50/70 penetration grade) and two highly modified polymer bitumens (PMB1 & PMB2), one with plastomer and one with elastomer, have been studied. The mixture samples had a continuous 0/10mm diorite grading, a 3 1% void content and a binder content of 6% by weight of the aggregate. 3. EXPERIMENTAL STUDY IN THE LINEAR VISCOELASTIC DOMAIN 3.1 Complex modulus tests on binders The complex modulus tests were performed at Total laboratory with a Metravib apparatus over a range of frequencies from 6.3 to 250Hz. From -30 to 30 C, compression/traction tests were conducted on cylindrical samples. From 30 to 60 C, hollow cylindrical samples were tested in annular shearing. The relation E* =3 G* (incompressibility) is supposed. It allows to plot the results in function of G* only. The two PMB s do not conform to the Time-Temperature Superposition Principle (TTSP): their Black curves are not unique (Fig. 4a). Yet, shear modulus master curves of G* can be plotted at a reference temperature, Fig. 3 - Tests performed on binders and mixes for each domain of behavior. T s. We call this property Partial Time-Temperature Superposition Principle (PTTSP) as the shifting procedure gives a unique master curve only for the norm of the modulus. As regards the two pure bitumens, Figs. 4a and 4b evidence a transition from a glassy plateau ( G* 0.7GPa) to a purely viscous behavior (phase angle 90 ). Moreover, Fig. 4c shows that the factors a T of the four bitumens are very close. This means that the shifts along the frequency scale with the temperature of the master curves are quasi-identical for these four bitumens. This result has been extended with a parallel study on these binders after two kinds of ageing (RTFOT & RTFOT+PAV) and with five other binders before and after ageing. 3.2 Creep tests on binders (Bending Beam Rheometer tests) Creep tests on bitumens (AASHTO TP1) were performed using a Bending Beam Rheometer at three different temperatures (-3;-9;-15 C for 10/20, -12;-18;-24 C for 50/70, -18.5;-21.5;-24.5 C for PMB1, -24.5;-27.5;-30.5 C for PMB2). Creep stiffness (S) and creep rate (m) were
F. Olard et al. / Materials and Structures 38 (2005) 121-126 123 Fig. 4 - (a) Black diagram, (b) G* master curves at T s =10 C, (c) Shift factors a T. Table 1 - BBR limiting temperatures (t=60s), T gg* (7.8Hz) and T ge* (7.8Hz) Binder T(S(60s)=300MPa) T(m(60s)=0.3) T gg* (7.8Hz) determined at six different loading times (t) ranging from 8 to 240s. The Superpave procedure temperatures at limiting creep stiffness (300MPa) and limiting m-value (0.3) determined at a loading time of 60 seconds, are presented in Table 1 (cf. section 3.4). Creep stiffness master curves can be constructed (Fig. 5a). It is then possible to obtain the shift factors b T which must be equal (if the hypothesis of a linear viscoelastic thermorheologically simple behavior can be applied to binders) to the inverse of factors a T, obtained from the complex modulus tests. Fig. 5b confirms that a T and 1/b T coefficients are very close. As 10/20 and 50/70 were not tested T ge* (7.8Hz) 10/20-12.2-14 -3.5 15 50/70-19.3-20.9-11 8 PMB1-19.3-21.6-11 6 PMB2-28.8-29.1-20 3.5 Fig. 5 - (a) S master curves at T s = 24.5 C (reference temperature), (b) Comparison between shift factors a T and 1/b T for T s =-24.5 C. as low as 24.5 C, their master curves and shift factors have been extrapolated. 3.3 Complex modulus tests on bituminous mixes An MTS press was used at Eurovia laboratory for a 10-3 to 30Hz range of frequencies, at low temperatures (+15; 0; -15; - 30 C). 220mm high cylindrical samples (diameter=80mm) were tested in compression/extension. The TTSP does not hold for the PMB1 and the PMB2 mixes since their Black curves are not unique (Fig. 6a). Nevertheless, stiffness modulus ( E* ) master curves can be plotted (Fig. 6b). As already observed for the PMB s, the PTTSP (cf. section 3.1) is also verified for the PMB1 and PMB2 mixes. Figs. 6c and 6d respectively show that the shift factors a T of the four mixes are very close to each other and that the shift factors of bitumens and mixes are highly correlated (previously evidenced by [3] and [4]). Thus, for the considered temperature range, the shift values along the frequency scale of the master curves are only due to the binders and quasiidentical for these four mixes. As Ferry [5] postulated universal constants C 1 and C 2 for polymers, it seems possible to consider, as a first approximation, universal constants for all bituminous materials. The appropriate coefficients C 1 =19 and C 2 =143 have also been fitted with five other unaged and aged bitumens, at a reference temperature T s =10 C. C 1 and C 2 are defined by the WLF equation which gives log(a T ) as a function of C 1, C 2, T s and T[ C] (Fig. 6d) : C1(T Ts ) log(a T ) C (T T ) 2 s (1)
124 F. Olard et al. / Materials and Structures 38 (2005) 121-126 m-value (m=0.3) are correlated with T gg* (7.8Hz) (respectively r 2 =0.999 and 0.997). This good correlation has been checked with five other bitumens. T gg* and T ge* are not very well correlated (r 2 =0.868). 4. EXPERIMENTAL STUDY IN THE FAILURE DOMAIN 4.1 SHRP direct tensile tests on binders As described in AASHTO TP3, 27mm high samples are elongated at 1mm/min and at a given temperature. At low temperatures, binders have a fragile behavior, whereas at high temperatures their behavior is ductile. From our results, we introduce a fragile/ductile transition temperature of binders at the studied strain rate, T fdb, which is considered to be the temperature at which the tensile strength peaks in the axes tensile strength-temperature (Fig. 7a). The value of T fdb is presented in Table 2 with the temperature corresponding to a strain of 1% at tensile strength, T( ( failure )=1%). 4.2 Direct tensile tests on mixes These tests were performed at constant temperatures between 5 C to 30 C, and with two different strain rates: 300 and 45000*10-6 m/m/h. The tensile strength only slightly depends upon the strain rate in the fragile domain so that, as a first approximation, the strength in the fragile domain can Fig. 6 - (a) E* Black diagram for mixes, (b) E* master curves at T s =15 C for mixes, (c) Mixes shift factors a T at T s =15 C, (d) Binders and mixes shift factors a T at T s =10 C. 3.4 Comparison between the tests in the linear viscoelastic domain Table 1 presents the BBR limiting temperatures (t=60s) along with the binders and mixes glass transition temperatures, T gg* and T ge*, which are respectively the extrapolated temperatures at the peak loss moduli G and E at 7.8Hz. The temperatures at BBR limiting creep stiffness (S=300MPa) and Fig. 7 - SHRP tensile tests results: (a) Binders tensile strength according to temperature, (b) Binders strain at maximum stress according to temperature.
F. Olard et al. / Materials and Structures 38 (2005) 121-126 125 Table 2 - Limiting temperatures in the failure domain Binder 10/20 50/70 PMB1 PMB2 T fdb -10-15 -20-30 T( ( failure )=1%) -11-18.5-19.5-32.5 T fdm (300*10-6 m/m/h) -11-19.5-25 -30.1 T fdm (45000*10-6 m/m/h) -3-13 -16-22.5 T failure (TSRST) -21.6-29.5-34.1-45.2 be determined whatever the chosen strain rate. Therefore, a high strain rate can be used in the fragile domain so as to gain time. In reference to the transition temperature concept presented for binders, we introduce the fragile/ductile transition temperature of bituminous mixes, T fdm, which varies up to 9 C, following the applied strain rate (Fig. 8). This pertinent and discriminating low-temperature parameter is reported in Table 2. 4.3 Thermal stress restrained specimen tests (TSRST) on mixes Restrained cooling tests were carried out at a cooling rate of 10 C/h from 5 C using an MTS press at Eurovia laboratory. The tests were run in duplicate or triplicate on 250mm high samples (diameter=60mm). In a complementary study, the coefficients of thermal contraction of the four mixes were measured from 5 C to 30 C, and vary from 30 to 15*10-6 m/m/ C. Therefore, the equivalent mechanical strain rate ranges from 300 to 150*10-6 m/m/h during the restrained cooling tests. As the material is restrained, its tendency to shorten results in the development of a thermal stress that produces failure. From our tests, failure occurs in the fragile domain when the thermal stress equals the tensile strength obtained at 300*10-6 m/m/h. This means that the strength of the bituminous mixes seems to be a function of the temperature [6] and the strain rate only, and does not depend upon the previous followed stress and temperature paths. Moreover, to the extent that the tensile strength only slightly depends on the strain rate in the fragile domain (Fig. 8), it seems possible to forecast the thermal cracking in the fragile domain by means of the tensile strength curve obtained at any strain rate (Fig. 9). Fig. 8 - Tensile strengths at constant temperatures of the four mixes. Fig. 9 - TSRST vs. tensile tests on mixes (data points) at 300*10-6 m/m/h. 4.4 Discussion on the failure properties of binders and mixes at low temperatures Table 2 gathers the following parameters: T fdb, T( ( failure )=1%), T fdm (300*10-6 m/m/h), T fdm (45000*10-6 m/m/h) and the failure temperature at the TSRST, T failure (TSRST).
126 F. Olard et al. / Materials and Structures 38 (2005) 121-126 T fdb and T( ( failure )=1%) are highly correlated with each other (r 2 =0.96). T fdm (300*10-6 m/m/h) and T fdm (45000*10-6 m/m/h) are correlated with T fdb (r 2 =0.95). Finally, the correlation between T( ( failure )=1%) and T failure (TSRST) is r 2 =0.97. 5. CONCLUSIONS A rational and pertinent approach which consists in comparing the properties of binders and mixes only in the same domain of behavior (either in the small strain amplitude domain or in the failure domain) was considered in this paper. In particular, in the small strain amplitude domain, it has been shown that it seems possible to define, as a first approximation, universal shift factors a T for all the bituminous materials (bitumens and mixes). The appropriate universal constants C 1 and C 2 of the WLF equation are proposed for the reference temperature T s herein considered. Parameters such as the BBR limiting temperatures, the fragile/ductile transition temperatures of binders and mixes and the failure temperature obtained in the TSRST tests, have been introduced. It has been shown that there is a good correlation of the low temperature parameters of binder and mixes, provided that the same domain of behavior is concerned. ACKNOWLEDGEMENTS The authors are thankful for the support that was provided by TotalFinaElf by furnishing the two pure bitumens and by performing the complex modulus tests on binders and the BBR tests on the two pure bitumens. Finally, the authors gratefully acknowledge the help provided by the staffs of Appia and Eurovia laboratories. REFERENCES [1] Olard, F., Étude et modélisation de comportement thermomécanique des enrobés bitumineux, Mémoire de recherche pour l obtention du Diplôme d Études Approfondies de Génie Civil de l École Doctorale de Lyon, 2000. [2] Di Benedetto, H., Nouvelle approche du comportement des enrobés bitumineux : résultats expérimentaux et formulation rhéologique, in Mechanical Tests for Bituminous Mixes, Characterization, Design and Quality Control, Proceedings of the Fourth RILEM Symposium, 1990. [3] Boutin, C., de La Roche, C., Di Benedetto, H. et Ramond, G., De la rhéologie du liant à celle de l enrobé bitumineux, Théorie de l homogénéisation et validation expérimentale, The Rheology of Bituminous Binders, European Workshop, Brussels, 5-7 April 1995. [4] Di Benedetto, H. and Des Croix, P., Binder-mix rheology: limits of linear domain, non linear behaviour, Eurobitume 1996. [5] Ferry, J.D., Viscoelastic properties of polymers, 3 rd Edn (John Wiley & Sons, 1980). [6] Arand, W., Behaviour of asphalt aggregate mixes at low temperatures, in Mechanical Tests for Bituminous Mixes, Characterization, Design and Quality Control, Proceedings of the Fourth RILEM Symposium, 1990.